How Do We Know Humans Are Causing Climate Change?

When I talk with people who are skeptical about climate change science, I'm often asked: "I know global warming is happening, but how do we know humans are causing the change?"

Broadly, this question requires a two-part explanation:

We know that carbon dioxide is the reason the Earth is warming.

We know that humans are sending huge amounts of carbon pollution into the atmosphere.

More than a century ago, scientists figured out that carbon dioxide and other greenhouse gases trap heat. Since then, we've identified many lines of evidence that confirm our planet is warming because of carbon dioxide and the greenhouse effect it produces. Plus, we have evidence that warming can't be attributed to other factors, like the sun. Solar activity has actually decreased in recent decades.

But how do we know we humans are behind the major increases in carbon dioxide? We've left many "fingerprints" that show this is the case.

Since the industrial revolution (the period when humans began intensely burning fossil fuels), levels of carbon dioxide in the atmosphere have increased by about 40%. During this same period, the ocean has become about 30% more acidic due to increased carbon dioxide dissolved in its waters. (The amount of carbon dioxide taken in by the upper layer of the ocean is increasing by about 2 billion tons a year!)

But this is no surprise. We're emitting carbon dioxide through the burning of fossil fuels faster than the earth can absorb it. And we have the ability to actually show that the excess carbon dioxide in our atmosphere and oceans is coming from fossil fuels, as opposed to natural sources like volcanoes.

When burned, fossil fuels combine with oxygen to produce energy and chemical byproducts like carbon dioxide. According to this recipe, as carbon dioxide is produced, oxygen is lost. Just as we might predict based on this chemistry, we're observing oxygen levels decrease in our atmosphere as carbon dioxide increases. This is a sign that carbon dioxide levels are increasing because we're burning fossil fuels.

There's more. Did you know that the carbon dioxide that comes from fossil fuels can be distinguished from other types of carbon dioxide? It tends to be a bit lighter. So analyses (looking at the carbon dioxide in air samples and the carbon stored in coral reefs) can pick out that the lighter type from fossil fuels is the kind of carbon dioxide that's increasing.

For an additional illustration of these two human fingerprints, check out this excellent snippet from Earth: The Operators' Manual. In it, geologist Richard Alley examines the evidence and concludes that the increase in carbon dioxide "is coming from our fossil fuels. It's us." Check out the full video here.